1. Field of the Invention
This invention refers to an electric nutrunner for tightening bolt joints to a desired tightening torque or clamping force.
2. Description of the Prior Art
In tightening a bolt joint two different phases may be observed. The first phase comprises the initial threading sequence, which can be said to be tightening without increasing the torque. In the second phase, the tightening sequence proper, the components of the joint are clamped together, and the torque will then have to increase continuously so as to make the tightening continue. These two phases are illustrated in FIG. 1, wherein the torque M is shown as a function of the angle of rotation .theta., with portion I referring to the initial threading sequence whereas portion II refers to the tightening sequence.
In mechanical tightening, i.e. tightening with a motorpowered nutrunner, the initial threading is carried out at a high angular velocity. During the tightening proper, the angular velocity of the driving pin will decrease progressively down to zero. This is illustrated in FIG. 2, wherein the angular velocity (d.theta.)/(dt) is shown as a function of time t, with the portion I representing the initial threading sequence whereas the portion II represents the tightening sequence.
Thus, during the tightening, the rotating parts of the nutrunner and the joint are braked, with the torque generated by the motor being supplemented by a deceleration torque -J(d.sup.2 .theta.)/(dt.sup.2), the magnitude of which is dependent of the inertia factor J of the rotating components and the rate of the deceleration -(d.sup.2 .theta.)/(dt.sup.2). Therefore, the torque delivered to the joint can generally be described by the following expression: EQU M.sub.F =M.sub.M -J(d.sup.2 .theta.)/(dt.sup.2) (1)
wherein
M.sub.F =the torque delivered to the joint, PA1 M.sub.M =the part of the torque delivered to the joint derived from the torque generated in the motor, PA1 J=the total of the rotational moment of inertia of all of the rotating components reduced to the driving pin of the machine, PA1 .theta.=the angle of rotation of the driving pin of the machine, and PA1 t=time. PA1 M.sub.n can be measured with great exactness, PA1 the term -J(d.sup.2 .theta./dt.sup.2).sub.n is minimized not only by a low value of J but also by compensating for the varying joint hardness by the control equipment, PA1 M.sub.x is minimized not only by the low value of J but also by a portion of the kinetic energy 1/2J(d.theta./dt).sup.2.sub.n being braked away.
In an ordinary pneumatic nutrunner the torque delivered by the motor is determined by the pressure of the supply air. In FIG. 3 the torque M is shown as a function of time t for a hard joint, with the relationship being indicated by characteristic I, and a soft joint, with which the relationship is illustated by characteristic II. Thus, the torque M.sub.F =M.sub.M -J(d.sup.2 .theta.)/(dt.sup.2) delivered to the joint will be dependent of the hardness of the joint as this hardness influences the dynamic additive torque -J(d.sup.2 .theta.)/(dt.sup.2), which is dependent of the rate of the braking. Said additive torque is substantial in hard joints but practically negligable in soft joints. The fact that the total torque M.sub.F is dependent of the hardness of the joint comprises a drawback whose elimination is being attempted in various ways.
One method of avoiding the above-mentioned drawback is to disconnect the motor drive at certain level of the delivered torque of the motor and if possible to select this level such; that it is adjusted to the specific joint hardness in each individual case. To accomplish this, a nutrunner is provided with some device for measuring the delivered torque directly or indirectly, and at a predetermined level M.sub.n turning off or disengagement of the motor is initiated. The tightening sequence of a hard joint will then have the appearence of FIG. 4, the upper portion of which illustrates the torque M as a function of time t, whereas its lower portion shows the angular velocity (d.theta.)/(dt) as a function of time. If t.sub.n designates the point at which disengagement or stopping of the motor is initiated, a torque of M.sub.n -J(d.sup.2 .theta./dt.sup.2).sub.n will have been delivered to the joint at that point. However, the final torque M.sub.s delivered to the joint will be greater. At the point t.sub.n, the motion of the motor will not have ceased, and the driving pin will have an angular velocity of (d.theta./dt).sub.n. Consequently, the rotating system will have a stored kinetic energy equal to W.sub.n =1/2J(d.theta..sup.2 /dt.sub.n).
Depending on the design of the system, this energy will in its entirety or in part be supplied to the joint as an additive torque M.sub.x.
Consequently, the resulting final torque M.sub.s will be EQU M.sub.s =M.sub.n -J(d.sup.2 .theta./dt.sup.2).sub.n +M.sub.x
In accordance with the invention, precise control of the tightening sequence is achieved by means of equipment consisting of a nutrunner and an assoicated control system up to a final torque of M.sub.s which is not affected by the hardness of the joint, said equipment being of such nature that
The above-mentioned precise control is achieved by utilizing an electric nutrummer having a low moment of inertia, furthermore a drive circuit for the motor of the nutrunner designed in such manner that the motor is braked by short-circuiting, wherein the tightening is interrupted, and a specific circuit in which the growth of the torque can be sensed in order that the additive torque -J(d.sup.2 .theta./dt.sup.2).sub.n may be compensated. As an example of a particularly appropriate motor a so-called permanently magnetized direct-current motor may be mentioned (PMLs motor), which has the characteristic that its delivered torque is directly proportional to the drive current, i.e. M.sub.motor =k.multidot.i, wherein k is a motor constant and i is the motor current. Thus it is simple to measure M.sub.motor by measuring the current, which for example may be expressed as the voltage drop over a resistor in series with the motor. By sensing the growth of the current (di)/(dt) during the tightening, for instance by measuring the voltage drop over an inductor in series with the motor, it becomes possible to compensate for the term -J(d.sup.2 .theta.)/(dt.sup.2 ). This will be described more specifically below.
As the result of new magnetic materials being rapidly developed, for example of the type of rare earth metal/cobolt, it has become possible to design motors having substantially smaller dimensions than with magnetic materials of the types presently in use.
By selecting a PMLs motor of the so-called moving coil type, i.e. a motor in which solely the winding and not the iron core rotates, a system having an extremely low moment of inertia is achieved, i.e. the term M.sub.x becomes small.